In most two-phase systems, the prime mover is a single-phase device, such as a compressor or pump, as pumping a two-phase mixture presents a challenge due to the compressibility of the vapor/gas phase. Various studies have been presented describing degraded pumping performance with two-phase flow under specific sets of conditions.
Centrifugal pumps can pump gas/vapor under certain conditions. As long as the thermodynamic quality of the vapor or gas (defined as the ratio of vapor or gas mass to total mass) is low, the pump may continue to operate, although with degraded performance. As the gas flow and quality increase, flow regime changes result in a separated flow within the pump and significantly reduced pumping capability.
The interest in developing oil resources in deeper water has resulted in some development of multiphase pumping such as the helico-axial multiphase pump, a form of inline rotary pump, which was introduced in the oil and gas industry in the 1990s. This pump uses a diffuser that breaks up large bubbles, which results in a more homogeneous flow. This type of flow allows the liquid momentum to be increased which in turn drags the gas along. Unfortunately, this limits the pumping capability to low quality two-phase flows.
Centrifugal or rotary devices are used in the power industry to raise the pressure head of condensate flows from the condenser to return to the boiler or nuclear reactor. These pumps require a hydrostatic column to provide enough net positive suction head. In addition, the pumps utilize multiple stages to limit the pressure rise per stage.
Gear pumps such as screw pumps, rotary gear pumps, and peristaltic pumps have been demonstrated to pump two-phase flows but only under a specific set of conditions, which limits their application. Typically, these devices can achieve neither high pumping head nor high quality flow.
Jet pumps or Bernoulli pumps have been used in various industries and can be found in vacuum systems, chemical dispensing, fuel cells, and thermal systems. These devices are driven by a nozzle operating at choked, or sonic, conditions. The high velocity flow exiting this nozzle is directed into a mixing chamber and exchanges kinetic energy with a lower pressure fluid. The mixed flows are then directed through a diverging stage that recovers pressure and results in an outlet pressure that lies between that of the inlet and driving flow. These pumps have been shown to pump vapor/gas but the pumps require significant motive flow rates and exhibit parasitic motive pressure drops. In addition, performance degrades significantly outside a narrow range of operation.
The oil and gas industry has relied upon a two-stage approach for pumping multiphase flows. The flow is first directed to a separator which produces separate single phase flow streams that can utilize standard pumps/compressors to raise the head of each phase. A similar approach was utilized with the Foster-Miller pump developed for the Air Force. This pump utilizes a turbine style separator to separate a refrigerant and directed the liquid to a rotating bowl that utilizes a Pitot pump to raise the liquid pressure and a separate centrifugal blower to raise the vapor pressure. The two separated phases, which are now at a higher pressure, can then be mixed as needed. However, this device requires a high power induction motor to provide the high speed rotation required for the Pitot pump and centrifugal blower. The pressure rise capable of this pump is dependent on a number of factors. For gas/vapor, the motor/blower speed and density of the fluid affects the discharge pressure. For liquid, not only does the motor speed affect the pump pressure but the amount of liquid in the bowl has an impact on pressure rise. Both of these effects are related to the hydrostatic pressure generated within the bowl and available to the Pitot pump.
Current aircraft electronics are cooled using conduction, forced-air cooling, or single phase liquid cooling. Thermal management by conduction alone can only be accomplished for low heat flux components that have access to a sufficiently cool sink, such as aircraft skin or fuel, within a short distance. For most electronics, cooling by conduction alone is not feasible and forced convection is necessary. The most accessible medium for convective cooling is air. However, air cooling is limited to low powers. As the power densities of aircraft electronics increase, air cooling becomes less attractive and liquid cooling is typically used. A liquid cooling system uses a single phase pumped loop with a liquid coolant, such as Polyalphaolephin (PAO) or aircraft fuel. Pumped liquid loops can transfer significantly more waste heat with lower thermal resistances than air cooling. The costs for these improvements are increased mass, power consumption, and system complexity.
Benefits associated with the integration of propulsion, electrical power, and Thermal Management Subsystems (TMS) have been identified. Associated with these benefits, however, are new thermal management challenges related to the collection, transport, and rejection of the waste heat produced by these subsystems. Subsystem integration results in higher power densities. Replacement of mechanical and analog systems with their digital counterparts results in aircraft that produce more waste heat than their predecessors. Higher power densities and compact assembly geometries make air cooling infeasible. Larger waste heat loads result in increased mass and power requirements for liquid pumped loops. As a result, issues with thermal management systems have been identified. New solutions that increase the effectiveness of both the component and aircraft thermal management systems are needed for the next generation of aircraft.
Current spacecraft thermal management involves single-phase pumped loops or passive, two-phase, capillary devices such as Constant Conductance Heat Pipes (CCHP), Capillary Pumped Loops (CPL), and Loop Heat Pipes (LHP). For low power applications, capillary devices are more attractive than single-phase systems due to their passive operation and use of latent heat transport, which allows for isothermal operation and less working fluid mass. Among these devices, CCHPs are the most common among satellite thermal management systems. However, CCHP systems are considerably design specific, which makes them difficult to interchange between applications and intolerant of design changes within an application. In addition, complex CCHP geometries are difficult to ground test. CPLs and LHPs, which share common operating principles, are somewhat more flexible as they are capable of pumping over significant distances using non-wicked transport lines and ground testing is not an issue. CPLs and LHPs have found application in several programs, such as the Hubble Space Telescope and Geoscience Laser Altimetry System. However, they are known to encounter start-up and transient anomalies which can lead to failure. To provide reliable start-up and de-priming recovery, electrical heaters or thermoelectric devices are often employed, which somewhat negates the passive advantage of capillary loops.
Moreover, all capillary devices share two disadvantages. First, they are restricted to relatively low powers of about a kilowatt or less, depending on wick design and available capillary head. This presents a problem considering increasing spacecraft thermal management demands. To meet higher powers, multiple capillary devices are needed. Second, capillary systems entail complicated fabrication processes related to wick performance and working fluid purity. These processes lead to higher fabrication and testing costs.
Single-phase pumped loops have been identified as a more flexible alternative to capillary systems in that they can operate with multiple heat sinks and sources, ground test well, require less analysis and design related to integration, and are capable of transporting considerably higher power. Single-phase systems have found application with the International Space Station, Mars Pathfinder, and Mars Exploration Rovers. However, these systems are actively pumped and, as such, require more power than passive devices such as CCHPs. Furthermore, since thermal energy is transported by sensible heating or cooling of the working fluid, these systems require higher mass flow rates, of approximately an order of magnitude or more, which affect pump size and power consumption, and larger system volume than two-phase systems for equivalent power levels. In addition, single-phase heat transfer is less efficient than condensation and evaporation, resulting in larger heat exchangers for single-phase systems.